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9068 



Bureau of Mines Information Circular/1986 



Dust Reduction Capabilities of Five 
Commercially Available Bag Valves 



By Andrew B. Cecala, Anthony Covelli, 
and Edward D. Thimons 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9068 



Dust Reduction Capabilities of Five 
Commercially Available Bag Valves 



By Andrew B. Cecala, Anthony Covelli, 
and Edward D. Thimons 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Model, Secretary 



BUREAU OF MINES 
Robert C. Horton, Director 



no, '^oG^ 




Library of Congress Cataloging in Publication Data: 



Cecala, Andrew B 

Dust reduction capabilities of five commercially available bag 
valves. 

(Information circular ; 9068) 

Supt. of Docs, no.: I 28.27: 9068. 

1. Ore-dressing plants — Dust control. 2. Valves. I. Covelli, 
Anthony. II. Tliimons, Hdward D. III. Title. IV. Series: Informa- 
tion circular (United States. Bureau of Mines) ; 9068. 



TN295.1J4 [TN5011 ()22s [622'.79l 85-600328 



CONTENTS 

Page 

Abstract 1 

Background 2 

Acknowledgments 4 

Testing 4 

Discussion 8 

Conclusion 10 

ILLUSTEiATIONS 

1. "Rooster tail" of product discharged from bag valve as bag falls from fill 

nozzle 2 

2 . Product accumulation on outside of bags 3 

3. Product that has escaped from bag valve during conveying process 4 

4. Sampling setup for field testing 5 

5. Sampling setup to monitor worker's dust exposure 6 

6. Dust reductions with extended polyethylene and foam valves compared with 

that of standard paper valve 8 

TABLES 

1. Results from initial 2-week test at various locations using 325-mesh 

product 7 

2. Results from final test at various locations using 200-mesh product 8 

3. Increase in valve cost above the cost of standard paper valve 9 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 




cfm 


cubic foot per minute min 


minute 


in 


inch mm 


millimeter 


lb 


pound pet 


percent 


mg/m^ 


milligram per cubic 
meter 





DUST REDUCTION CAPABILITIES OF FIVE COMMERCIALLY 
AVAILABLE BAG VALVES 

By Andrew B. Cecala,^ Anthony Covelli,^and Edward D. Thimons^ 



ABSTRACT 

The dust-reduction capabilities of five commercially available bag 
valves were evaluated by the Bureau of Mines for use in mineral process- 
ing plants. The five valves were made of (1) standard paper, (2) poly- 
ethylene, (3) extended polyethylene, (4) double trap (p^^per) , and (5) 
foam. The valves were evaluated at a mineral processing plant during 
the bag loading, conveying, and palleu loading process. For the overall 
process, the extended polyethylene valve was the most effective at re- 
ducing product blowback, and resulted in lower dust concentrations for 
bag-generated dust. When compared to the standard paper, it gave a 62- 
pct reduction in respirable dust at the bag operator location, and a 
66-pct reduction at the bag stacker location. The cost of the extended 
polyethylene valve is less than 1 cent per bag higher than that of the 
standard paper valve. The high dust reductions and slight cost increase 
make it an attractive choice for the mineral processing industry. 



'Mining engineer. 
^Physical scientist technician. 
-^Supervisory physical scientist. 
Pittsburgh Research Center, Bureaii of Mines, Pittsburgh, PA. 



BACKGROUND 



The intent of this Bureau of Mines work 
was to determine the dust reduction capa- 
bilities of various bag valves available 
to the mineral processing industry. 

Many different types of ground mineral 
products are packaged into 50- or 100- 
Ib paper bags. Although bag technology 
is continually improving, contamination 
from bags still remains a significant 
source of dust exposure for workers. 
Most of the bag contamination comes from 
the bag valve. The bag valve is simply a 
means of inserting the fill nozzle into 



a bag. When the bag is full and falls to 
the conveyor, the product within the bag 
forces the valve closed. The valve usu- 
ally does not close properly, however, 
because product is usually trapped inside 
it during filling and ejection. As a re- 
sult, the valves leak product, and, 
therefore, dust is generated during the 
conveying and pallet-loading process. 
This dust contaminates the work environ- 
ment and is a health hazard to persons 
working in the area. 




P 



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FIGURE 1. - "Rooster tail" of product discharged from bag valve as bag falls from fill nozzle. 



Bag-generated dust contaminates the 
work area in a number of ways. One 
source is product blowback during the 
fill cycle. During filling, the inside 
of the bag becomes pressurized by the 
fluidizing air, which is used to carry 
the product into the bag. This pressure 
is relieved by air and product blowing 
out of the bag valve. This produces a 
considerable amount of airborne dust to 
which the bag operator is exposed. When 
the bag has finished filling and falls 
from the fill station, a "rooster tail" 
of product is discharged from the valve 
(fig. 1). As the bag hits the conveyor 



belt, additional product is discharged 
from the valve, further contaminating the 
work environment and exposing the bag 
operator. 

These dust sources 
to the accumulation of 
outside of the bag (fig. 
of product are conveyed 
loading area, this accumulation is dis- 
persed into the air, exposing workers, 
especially the bag stackers. In addi- 
tion, as the bags drop from one conveyor 
to another or from the conveyor to the 
bag slinger, dust is again emitted from 
the valve (fig. 3) . 



also contribute 

product on the 

2) . As the bags 

to the pallet- 




I' 



FIGURE 2. - Product accumulation on outside of bags. 





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rC-i-C*"'^ < * f * tf * t t * * ? '^ ' <• f-r 



FIGURE 3. - Product that has escaped from bag valve during conveying process. 



ACKNOWLEDGMENTS 



The authors thank the members of the 
National Industrial Sand Association's 
Committee on Engineering and Technology 
for its guidance and cooperation. Spe- 
cial thanks also to James P. Snider, Vice 
President and General Manager of Central 



Silica Co. , for his cooperation and as- 
sistance in the actual testing at one of 
the Central Silica facilities. We would 
also like to thank Jon C. Volkwein, Bu- 
reau of Mines, for helping to establish 
this project. 



TESTING 



A 3-week study was performed to evalu- 
ate five different bag valves that were 
considered either to be the most common- 
ly used, or to be potentially the most 
effective. Initially, 2 weeks of testing 
were performed to compare the following 
five bag valves: 

1. Standard paper (used as the normal). 

2. Polyethylene. 

3. Extended polyethylene, 

4. Double trap (paper). 

5. Foam. 

The standard paper and polyethylene 
valves are the two most common valves 



used in the mineral processing indus- 
try. The last three valves were believed 
to have the greatest dust reduction 
potential. 

In preparation for testing, the con- 
veyor and pallet-loading area were en- 
closed in a tunnel made of wood framing 
and thick plastic. A 1,300-cfm blowing 
fan directed air through this enclosure. 
By knowing the dust concentration coming 
into the enclosure, the amount of dust 
generated from bag contamination and 
valve leakage could be determined. The 
enclosure and fan provided a controlled 
atmosphere and reduced contamination from 
outside sources. 



Five real-time aerosol monitors (RAM) 
were placed at various locations through- 
out the bagging and pallet-loading area 
to measure respirable dust concentra- 
tions. The RAM-1 monitors, developed by 
GCA Corp. 3 under a Bureau contract, use a 
light-scattering device to calculate the 
dust concentration of a sample drawn in 
from the environment. The monitors can 
be sensitive to changes in the dust con- 
tent (size, shape, refractive index), 
but if calibrated to a specific dust 
content, their accuracy is within ±10 pet 
of gravimetric samplers equipped with 
the standard 10-mm cyclone to measure 
respirable dust.^ Before the RAM-1 moni- 
tors were taken into the field, they were 



Reference to specific manufacturers 
does not imply endorsement by the Bureau 
of Mines. 

^VTilliams, K. L. , and R. J. Timko. 
Performance Evaluation of a Real-Time 
Aerosol Monitor. BuMines IC 8968, 1984, 
20 pp. 



calibrated side by side in a dust box. 
The RAM-1 dust monitors were used in 
this study for a comparative evaluation. 
Since the dust concentration for each 
valve type was compared to that of each 
of the other four valves, it was not nec- 
essary to use the specific dust type for 
calibration. The output from these dust 
monitors were continuously recorded on a 
data logger and dual-pen strip-chart re- 
corders. The following sampling posi- 
tions were monitored (fig. 4) : 

Intake. — Located upstream of the blower 
to measure intake dust concentrations. 

Operator. — Located above the bag opera- 
tor to measure the dust in the bag pit 
area. 

Transfer. — Located slightly downstream 
from the conveyor transfer point to mea- 
sure dust generated at that point. 

Right and left stacker. — A cyclone at- 
tached to lapel of each bag stacker to 
give a direct indication of their dust 
exposure. 



Stackers 




KEY 

• Sample location 

-^Airflow 

— Enclosure 



Transfer 



Bag 

room 

intake 




'^f^ — 



L_ 



{ 



Intake 



% 



7 / / / / -7-} 



\ 



Operator 



fir/j7illI iii/J7///7 -7-r 



FIGURE 4. - Sampling setup for field testing. 



In all cases, the 10-mm cyclone was 
physically separated from the monitor and 
connected by Tygon vinyl tubing. To mon- 
itor a worker's dust exposure, a cyclone 
was attached to the worker's lapel, as 
shown in figure 5. The cyclone was con- 
nected to the dust monitor by the vinyl 
tubing to allow the worker to perform his 
job function with minimal interference. 
Since the tubing length at each sample 
location remained the same, deposition on 
the inner tubing walls was neglected, 
especially since the monitors were used 
for a comparative evaluation in comparing 
one valve type to another. 

Each valve type was tested separately 
on a truck-by-truck basis (480 bags). 
After one truck was loaded, the bags were 
changed to a different valve type, until 
each valve type had been tested; then the 
cycle was repeated. At the conclusion of 
the 2-week test period, the data were an- 
alyzed for each valve type. 

The results of the 2-week test are 
shown in table 1 for 325-mesh product. 
The values in the table represent a time- 
weighted average concentration for the 
dust measurements at each location. The 
measured dust concentration for each 
truck was multiplied by the run time. 
The value for each truck was then summed 
to give the average concentration for 
each monitoring location. The intake 
dust concentrations were subtracted for 
each of the sampling locations. The mea- 
sured dust concentrations for each posi- 
tion were ranked from one to five (lowest 
to highest). These ranking values were 
totaled to indicate an overall compara- 
tive rating for each valve. The lower 



the number, the more effective the valve 
was at reducing bag-generated dust. The 
results from the foam valve were limited 
to one sampling period because the valve 
was undersized, which caused difficulty 
in placing the bag on the nozzle, 
and would not allow the bag to eject 
automatically after filling. This under- 
sized valve created production delays and 
prevented additional testing of the foam 
valve. 

The extended polyethylene valve showed 
the greatest dust reduction during the 
conveying and pallet-loading process. 
The foam valve also gave substantial dust 
reductions, but it must be remembered 
that this testing was limited. Dust con- 
centrations with the double trap and the 
polyethylene valves were higher than with 
the standard paper valve except at the 
bag stacker location for the polyethylene 
valve. 

Based on these results, it was decided 
to perform some additional testing on the 
standard paper, the extended polyethyl- 
ene, and the foam valves. Additional 
foam valve bags were acquired with the 
correct valve size. Due to financial 
contraints by the cooperating plant for 
this study, it was only possible to order 
500 bags with foam valves. Therefore, 
the results of this final analysis are 
based on one truckload (480 bags) of each 
valve type (standard paper, extended 
polyethylene, and foam), which takes ap- 
proximately 50 min to load. To eliminate 
fluctuations due to different work pat- 
terns and style, each employee at the 
ground-silica-bagging area was asked to 
work the same position for the entire 



TABLE 1. - Results from initial 2-week test at various locations using 325-mesh 
product 



Valve 


Operator 


Transfer 


Bag stackers 


Total of 




Cone, mg/m^ 


Rank 


Cone, mg/m^ 


Rank 


Cone, mg/va? 


Rank 


rankings 


Double trap 


0.20 
.04 
.02 
.24 
.05 


4 
2 
1 
5 
3 


0.67 
.15 
.25 
.47 
.41 


5 
1 
2 
4 
3 


0.76 
.32 
.56 
.46 
.55 


5 
1 
4 
2 
3 


14 


Extended polyethylene 
Foam 


4 
7 


Polyethylene 


11 


Standard paper 


9 



NOTE. — Intake dust levels have been subtracted from measured dust concentrations. 




FIGURE 5. - Sampling setup to monitor worker's dust exposure. 



test. An additional sample location was 
included and one sample location was im- 
proved, as follows: 

Operator. — The cyclone was relocated to 
the bag operator's lapel to give a direct 
indication of dust exposure. 

Bag room intake. — The cyclone was lo- 
cated at the intake window into the bag 
room to indirectly measure the amount 
of dust liberated during the conveying 
process. 

Due to truck schedules the week of test- 
ing, it was more advantageous to evaluate 
the bag valves using 200-mesh product 
than the 325 mesh used for the 2-week 
evaluation. 

The results of this final analysis are 
shown in table 2. Because the intake 
concentrations did not fluctuate sig- 
nificantly during the 2-week evaluation 
and because of a faulty pump in a RAM-1 
dust monitor, the intake sample location 
was eliminated, and thus the intake con- 
centrations were not subtracted as in the 
first test; thus, the values are the ac- 
tual recorded concentrations. The ex- 
tended polyethylene valve had the lowest 
dust concentrations at all sample* loca- 
tions, and the foam valve showed an im- 
provement over the standard paper valve 
at each location. Figure 6 shows the 
percent dust reduction with these two 
valves over that of the standard paper 
valve. The two most critical sample lo- 
cations, those of the bag operator and 
the bag stacker, showed a 62-pct and a 
66-pct reduction, respectively, using the 
extended polyethylene, as compared with a 
49-pct and a 35-pct reduction, respec- 
tively, for the foam valve. 




Transfer Operator 



Bag room 
intake 



Bag 
stackers 



SAMPLE LOCATIONS 

FIGURE 6. - Dust reductions with extended poly- 
ethylene and foam valves compared v/ith that of stan- 
dard paper valve. 

TABLE 2. - Results from final test at 
various locations using 200-mesh 
product, milligrams per cubic meter 



Valve 


Oper- 
ator 


Trans- 
fer 


Bag 
room 
intake 


Bag 
stack- 
ers 


Extended 

polyethylene. 
Foam 


0.36 
.48 
.94 


0.69 

.73 

1.28 


0.28 
.35 
.66 


1.03 
1.94 


Standard paper 


2.99 



NOTE. — Intake dust levels have not 
been subtracted from measured dust 
concentrations . 



DISCUSSION 



The significant portion of bag- 
generated dust begins in the bag valve 
area. This leakage from the bag valve 
either directly contaminates the work 
environment, or soils the outside of 
the bag, which contaminates the work 



environment later during the conveying 
and pallet-loading process. The effec- 
tiveness of the different bag valves ap- 
pears to depend on two factors: the 
valve length and the material. 



The first factor that influences the 
effectiveness of each valve type is 
the length of the valve. The lengths 
of the valves used for this study are 
as follows, in inches: 

Double trap 4.5 

Extended polyethylene... 6.0 

Foam 4.0 

Polyethylene 4.5 

Standard paper 5.5 

For the same valve material, the longer 
the valve, the more effective it is at 
reducing leakage. This is most clearly 
demonstrated by the substantial differ- 
ence between the extended polyethylene 
and the normal polyethylene valve. The 
additional 1.5 in of length significantly 
reduced leakage from the valve, and thus 
bag-generated dust. However, the valve 
should never be longer than the fill noz- 
zle because this could substantially in- 
crease product blowback. 

The second factor that influences the 
valve effectiveness is the material used 
in the valve: namely, paper, polyethyl- 
ene, and foam. Polyethylene is a plastic 
material, lighter than paper, and thus 
may provide for a more effective seal. 
The idea behind the foam valve is that 
the open cells in the foam allow excess 
air to exit the bag but retaining the 
product. If the same bag valve length 
were tested for each valve material, 
the speculated ranking of material in 
this test would be foam, polyethylene, 
and paper, from the most to the least 
effective. 

The foregoing two factors justify the 
order of effectiveness in the bag valve 
testing. For instance, the double trap, 
which was initially expected to be an ef- 
fective valve, proved to be the least ef- 
fective, because it was the second short- 
est valve and made of what we believe was 
the least effective material (paper). 
Another example is the foam valve, which 
was ranked as the second most effective 
valve. Foam is believed to be the most 
effective material, but the valve was the 
shortest one tested. If the foam valve 



were lengthed by an inch or two, it might 
be more effective than even the extended 
polyethylene valve. 

A major consideration in selecting a 
specific valve is the increase in cost 
over that of the paper valve. A number 
of factors influence the cost of a valve. 
The first is the inherent cost of the ma- 
terial and the quantity of material re- 
quired to make the valve. The second 
factor would be the amount of work neces- 
sary to make the valve, such as folding 
it in a specific way and the difficulty 
in actually gluing it in place. Table 3 
shows the increased cost of the different 
valves over that of the standard paper 
valve. These costs represent the average 
of prices obtained from three different 
manufacturers, with the exception of the 
foam valve, which is manufactured by only 
two companies at the present time. 

When a company selects a valve type, it 
chooses the most cost effective valve 
possible, which incorporates the effec- 
tiveness of the valve in reducing product 
blowback and bag-generated dust and the 
cost of the valve. These tests have 
shown that for the five bag valves test- 
ed, tne extended polyethylene valve ap- 
pears to be the most effective valve for 
reducing product blowback from the bag 
valve, thus resulting in lower dust 
concentrations for bag-generated dust for 
the conveying and pallet-loading process. 
It also has the lowest cost increase (the 
same as the normal polyethylene) over 
that of the standard paper valve. The 
extended polyethylene, therefore, was the 
most cost-effective valve tested. 

TABLE 3. - Increase in valve cost above 
the cost of standard paper valve 





Price per 


Price 


Valve 


1,000 bags. 


per bag, 




dollars 


cents 


Polyethylene 


6.85 


0.7 


Extended 






polyethylene 


6.85 


.7 


Double trap 


11.17 


1.1 


Foam 


214.98 


21.5 



686 277 



10 



CONCLUSION 



An evaluation was performed to deter- 
mine the effectiveness of five commer- 
cially available bag valves in reducing 
dust generated during bag filling, con- 
veying, and pallet loading. The five 
valves tested included standard paper, 
polyethylene, extended polyethylene, dou- 
ble trap, and foam. Two factors appeared 
to determine the effectiveness of these 
bag valves. The first was valve length, 
in which the longer the valve, the more 
effective it was in reducing product 
blowback and bag-generated dust. The 
second factor was the valve material. It 
is speculated that the foam was the most 



effective material for reducing dust gen- 
eration, followed by the polyethylene, 
and then standard paper. Considering 
both factors , the extended polyethyl- 
ene was the most effective valve tested, 
and resulted in a 62-pct reduction in 
respirable dust exposure for the bag op- 
erator, and a 66-pct reduction for the 
bag stackers. The extended polyethylene 
was also the most cost-effective bag 
valve tested. The increase in cost for 
the extended polyethylene valve is ap- 
proximately $6.85 per thousand bags (0.7 
cent per bag) over that of the standard 
paper valve. 



■i us GOVERNMENT PRINTING OFFICE 1966-605-01 7/40,008 



INT.-BU.O F MIN ES,PGH.,P A. 28217 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh, Pa. 15236 



AN EQUAL OPPORTUNITY EMPLOYER 



OFFICIAL BUSINESS 

PENALTY FOB PRIVATE USE. $300 

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